Myocardial contrast echocardiography can be used to assess the microvascular response to vascular endothelial growth factor-121

Effect of Thrombus Composition and Viscosity on Sonoreperfusion Efficacy in a Model of Micro-Vascular Obstruction

Distal embolization of micro-thrombi during stenting for myocardial infarction causes micro-vascular obstruction (MVO). We have previously shown that sonoreperfusion (SRP), a microbubble (MB)-mediated ultrasound (US) therapy, resolves MVO from venous micro-thrombi in vitro in saline. However, blood is more viscous than saline, and arterial thrombi that embolize during stenting are mechanically distinct from venous clot. Therefore, we tested the hypothesis that MVO created with arterial micro-thrombi are more resistant to SRP therapy compared with venous micro-thrombi, and higher viscosity further increases the US requirement for effective SRP in an in vitro model of MVO. Lipid MBs suspended in plasma with adjusted viscosity (1.1 cP or 4.0 cP) were passed through tubing bearing a mesh with 40-μm pores to simulate a micro-vascular cross-section; upstream pressure reflected thrombus burden. To simulate MVO, the mesh was occluded with either arterial or venous micro-thrombi to increase upstream pressure to 40 mmHg ± 5 mmHg. Therapeutic long-tone-burst US was delivered to the occluded area for 20 min. MB activity was recorded with a passive cavitation detector. MVO caused by arterial micro-thrombi at either blood or plasma viscosity resulted in less effective SRP therapy compared to venous thrombi. Higher viscosity further reduced the effectiveness of SRP therapy. The passive cavitation detector showed a decrease in inertial cavitation when viscosity was increased, while stable cavitation was affected in a more complex manner. Overall, these data suggest that arterial thrombi may require higher acoustic pressure US than venous thrombi to achieve similar SRP efficacy; increased viscosity decreases SRP efficacy; and both inertial and stable cavitation are implicated in observed SRP efficacy.

Echocardiographic evaluation of the effects of stem cell therapy on perfusion and function in ischemic cardiomyopathy

BACKGROUND

Small animal models of ischemic left ventricular (LV) dysfunction are important for the preclinical optimization of stem cell therapy. The aim of this study was to test the hypothesis that temporal changes in LV function and regional perfusion after cell therapy can be assessed in mice using echocardiographic imaging.

METHODS

Wild-type mice (n = 25) were studied 7 and 28 days after permanent ligation of the left anterior descending coronary artery. Animals were randomized to receive closed-chest ultrasound-guided intramyocardial delivery of saline (n = 13) or 5 × 10(5) multipotential adult progenitor cells (MAPCs; n = 12) on day 7. LV end-diastolic and end-systolic volumes, LV ejection fraction, and stroke volume were measured using high-frequency echocardiography. Multiplanar assessments of perfusion and defect area size were made using myocardial contrast echocardiography.

RESULTS

Between days 7 and 28, MAPC-treated animals had 40% to 50% reductions in defect size (P < .001) and 20% to 30% increases in total perfusion (P < .01). Perfusion did not change in nontreated controls. Both LV end-diastolic and end-systolic volumes increased between days 7 and 28 in both groups, but LV end-systolic volume increased to a lesser degree in MAPC-treated compared with control mice (+4.2 ± 7.9 vs +19.2 ± 22.0 μL, P < .05). LV ejection fraction increased in the MAPC-treated mice and decreased in control mice (+3.0 ± 4.3% vs -5.6 ± 5.9%, P < .01). There was a significant linear relation between the change in LV ejection fraction and the change in either defect area size or total perfusion.

CONCLUSIONS

High-frequency echocardiography and myocardial contrast echocardiography in murine models of ischemic LV dysfunction can be used to assess the response to stem cell therapy and to characterize the relationship among spatial flow, ventricular function, and ventricular remodeling.

Trends in myocardial contrast echocardiography: parameteric versus volumetric imaging

It is now possible to perform myocardial contrast echocardiography at the bedside with an intravenous injection of commercially available contrast media. Although myocardial contrast echocardiography is a sensitive method for the detection of coronary stenosis and myocardial viability, its diagnosis has relied largely on the subjective interpretation of regional perfusion by experienced clinicians. Thus, quantification of myocardial contrast echocardiography data and displaying comprehensive images have been necessary for its routine application. In this review, new methods for quantifying or displaying myocardial contrast echocardiography parameters will be introduced: firstly, parametric imaging that displays the parameters of myocardial blood volume, blood flow velocity and myocardial blood flow separately; and secondly, color-coded maps of myocardial blood volume established from one myocardial contrast echocardiography image. These quantitative techniques can provide comprehensive and easy-to-understand images, although the quality of the baseline image remains a critical factor.

Imaging myocardial metabolic remodeling

Myocardial metabolic remodeling is the process in which the heart loses its ability to utilize different substrates, becoming dependent primarily on the metabolism of a single substrate such as glucose or fatty acids for energy production. Myocardial metabolic remodeling is central to the pathogenesis of a variety of cardiac disease processes such as left ventricular hypertrophy, myocardial ischemia, and diabetic cardiomyopathy. As a consequence, there is a growing demand for accurate noninvasive imaging approaches of various aspects of myocardial substrate metabolism that can be performed in both humans and small-animal models of disease, facilitating the crosstalk between the bedside and the bench and leading to improved patient management paradigms. SPECT, PET, and MR spectroscopy are the most commonly used imaging techniques. Discussed in this review are the strengths and weaknesses of these various imaging methods and how they are furthering our understanding of the role of myocardial remodeling in cardiovascular disease. In addition, the role of ultrasound to detect the inflammatory response to myocardial ischemia will be discussed.

Imaging myocardial angiogenesis

Imaging myocardial angiogenesis presents a major technical challenge because the ideal spatial resolution required is substantially higher than that available with standard X-ray angiography and nuclear medicine imaging. Moreover, these clinical imaging methods are currently inadequate (because of insufficient resolution) for clinical trials of angiogenic agents for the treatment of ischemic heart disease. Specialized techniques in MRI, ultrasonography, echocardiography and CT that are under development might provide improved means of imaging myocardial angiogenesis. Molecular imaging technologies are also being developed to improve resolution and to provide a better mechanistic insight into angiogenic therapies for ischemic heart diseases. This Review examines advanced methods for imaging angiogenesis. These technologies might soon permit data to be obtained directly from scientific studies and clinical trials.

The application of microbubbles for targeted drug delivery

Interest in microbubbles as vehicles for drug delivery has grown in recent years, due in part to characteristics that make them well suited for this role and in part to the need the for localized delivery of drugs in a number of applications. Microbubbles are inherently small, allowing transvascular passage, they can be functionalized for targeted adhesion, and can be acoustically driven, which facilitates ultrasound detection, production of bioeffects and controlled release of the cargo. This article provides an overview of related microbubble biofluid mechanics and reviews recent developments in the application of microbubbles for targeted drug delivery. Additionally, related advances in non-bubble microparticles for drug delivery are briefly described in the context of targeted adhesion.

Imaging of angiogenesis in cardiology

In the past decade, there have been major improvements in our understanding of angiogenesis at the genetic, molecular and cellular levels. Concentrated efforts in this area have led to new therapeutic approaches to ischaemic heart disease using angiogenic factors, gene therapy and progenitor cells. Despite very promising experimental results in animal studies, large clinical trials have failed to confirm the results in patients with coronary artery disease. Important questions such as selection of growth factors and donor cells, as well as the timing, dose and route of administration, have been raised and need to be answered. Molecular imaging approaches which may provide specific markers of the angiogenic process (e.g. integrin expression in endothelial cells) have been introduced and are expected to address some of these questions. Although few clinical imaging results are currently available, animal studies suggest the potential role of molecular imaging for characterisation of the angiogenetic process in vivo and for the monitoring of therapeutic effects.

Gated SPECT imaging to detect changes in myocardial blood flow during progressive coronary occlusion

BACKGROUND

The ability to track dynamic changes in myocardial blood flow (MBF) and wall motion with serial gated perfusion imaging may be a limiting factor in assessing new therapies. The purpose of this study was to determine whether gated Tc-99 m sestamibi (MIBI) SPECT imaging can track small changes in MBF in a model of progressive ischemia.

METHODS

Eight pigs (20 kg) underwent lateral thoracotomy for placement of an ameroid constrictor on the left circumflex coronary artery (LCX) and indwelling femoral and left atrial catheters for serial microsphere determinations of absolute MBF. Animals underwent concurrent left atrial microsphere and Tc-99 m sestamibi (0.3 mCi/Kg IV) injections at weekly intervals over 6 weeks per animal. Gated SPECT imaging was acquired for each injection using high resolution collimation and standard processing. The animals were sacrificed on day 42. Mean signal intensity (SI) from regions of interest (ROI) corresponding to control and ischemic MBF by microspheres was measured for three SPECT short-axis images. Mean contrast ratio (MCR) was calculated from the ratio of ischemic to control SI per slice. Regional wall motion (RWM) from gated images was scored 1-5 using a 16 segment model and a score index (RWMI) was calculated.

RESULTS

MBF decreased progressively (27% below resting values [P < 0.0001]) but with a clear and significant partial recovery by day 42 (13% improvement from peak ischemia, [P < 0.01]). SPECT perfusion and gated RWM closely paralleled the dynamic pattern of MBF caused by the ameroid constrictor. SPECT MCR decreased 21% from baseline scans in the LCX territory (P < 0.0001) and improved 11% from peak ischemia (P < 0.01) while the gated RWMI (1.0 at baseline) peaked at 1.36 and improved to 1.13 by day 42.

CONCLUSION

Gated SPECT-a technique readily available-tracks dynamic changes in MBF closely with both perfusion and RWM. For trials of new therapies for the alleviation of chronic ischemia, these findings have direct implications for measuring efficacy.

Angiogenesis therapies for cardiovascular disease

PURPOSE OF REVIEW

The purpose was to summarize the findings of the proangiogenic clinical trials using protein and gene therapy, with analysis of the problems and an interpretation of the results.

RECENT FINDINGS

Recent findings include several new large clinical trials, using both gene and protein therapies. There has been development of new basic science concepts, especially with regard to endothelial activation and stabilization of newly formed microvessels. This review provides a critical analysis of the most recent clinical trials, both in efforts to understand the pitfalls of earlier clinical trials, and also to focus on requirements for future studies.

SUMMARY

This article reviews many of the clinical trials utilizing proangiogenic therapy, assesses the pitfalls seen within the current trials, and discusses the conclusions drawn and the future of angiogenesis therapy.

Assessment of ischemia-induced microvascular remodeling using contrast-enhanced ultrasound vascular anatomic mapping

Our aim was to apply novel contrast-enhanced ultrasound (CEU) techniques to characterize remodeling in different vascular compartments during ischemia-mediated angiogenesis. Hind limb ischemia was produced by ligation of an external iliac artery in 60 rats, half of which were treated with intramuscular fibroblast growth factor (FGF)-2 (5 microg). The proximal adductor muscles of the ischemic and control hind limb were studied immediately after ligation and at days 4, 7, or 14. Low-power maximum intensity projection imaging was performed to assess large intramuscular vessels to the fourth branch order. CEU data were analyzed to measure capillary perfusion and functional noncapillary microvascular blood volume. Resting capillary perfusion was reduced by 30% after arterial ligation and recovered earlier in FGF-2-treated versus nontreated rats (day 4 vs. 14). Changes in perfusion were temporally related to expansion of noncapillary microvascular blood volume on CEU, which was associated with an arteriogenic response on histology. Expansion of and organization (fractal distribution) of large collateral vessels occurred gradually over 2 weeks and was slightly more rapid with FGF-2 treatment. We conclude that CEU can separately assess collateral development, more distal arteriogenesis, and secondary changes in capillary perfusion that occur differentially with ischemia and growth factor therapy.

Contrast echocardiography

Myocardial contrast echocardiography (MCE) is a noninvasive imaging technique that relies on the ultrasound detection of microbubble contrast agents. These agents are confined to the intravascular space thereby producing signal enhancement from the blood pool. This review encompasses many of the key concepts regarding the clinical application of MCE. The first section focuses on the composition, safety, and biokinetics of ultrasound contrast agents. Then we discuss new ultrasound imaging methodology that has been developed to enhance detection of contrast agent and to assess perfusion at the tissue level. Next, the clinical applications of contrast ultrasound are reviewed. These include enhancement of the cardiac chambers for better assessment of cardiac function and masses, myocardial perfusion imaging for the detection of coronary artery disease, and the assessment of myocardial viability and microvascular reflow. Finally, we discuss some of the future applications for MCE, which include molecular imaging of disease and drug/gene delivery. The overall aim of the review is to update the clinician on state-of-the-art MCE and how it can be applied in patients with cardiovascular disease.

Gene and cell therapy for chronic ischaemic heart disease

Viable treatment options are becoming available for the 'no-option' patient with chronic ischaemic heart disease. Instead of revascularising the highly diseased epicardial coronary arteries, scientists and clinicians have been looking at augmenting mother nature's way of providing biological bypass in an attempt to provide symptomatic relief in these patients. The novel use of gene and cell therapies for myocardial neovascularisation has exploded into a flurry of early clinical trials. This translational research has been motivated by an improved understanding of the biological mechanisms involved in tissue repair after ischaemic injury. While safety concerns will be top in priority in these trials, different types or combination of therapies, dose and route of delivery are being tested before further optimisation and establishment. With cautious optimism, a new era in the treatment of ischaemic heart disease is being entered. This article reviews the present state in gene and cell therapies for ischaemic heart disease, the modalities of their delivery, novel imaging techniques and future perspectives.

Effects of Long-Term Oral Dipyridamole Treatment on Coronary Microcirculatory Function in Patients With Chronic Stable Angina: A Substudy of the Persantine In Stable Angina (PISA) Study

AIMS

A meta-analysis of 13 randomized placebo-controlled trials demonstrated a benefit for dipyridamole therapy, particularly with longer duration of treatment. Although the mechanism of this effect is not well understood, dipyridamole increases endogenous tissue adenosine, which may have a beneficial effect on myocardial perfusion. Therefore, we measured the effects of dipyridamole on myocardial blood flow (MBF) and coronary flow reserve (CFR) by using positron emission tomography and H2O in patients with coronary artery disease.

METHODS

Forty-four patients with angiographically documented coronary artery disease were double-blind randomized to either oral dipyridamole [200 milligrams (mg) twice daily (bd)] or placebo as add-on to conventional antianginal treatment for 24 weeks. MBF was measured at rest and during dobutamine stress at baseline and study completion for the region subtended by the most severe coronary artery stenosis (Isc) and remote myocardium subtended by arteries with minimal or no disease (Rem). CFR was calculated as MBF-peak/MBF-rest.

RESULTS

Thirty-five patients completed the study. Isc MBF-rest decreased in patients receiving dipyridamole (0.10 mL/minute/g; P = 0.03) and increased in the placebo group (0.16 mL/minute/g; P = 0.01) during the 24-week study. No significant change in MBF-peak was demonstrated in either group. Consequently, Isc-CFR increased significantly in patients receiving dipyridamole (1.65 +/- 0.47 vs 1.83 +/- 0.67; P < 0.05). By contrast, Isc-CFR decreased significantly in those receiving placebo (1.74 +/- 0.44 versus 1.38 +/- 0.46; P < 0.03). No change was seen in Rem-CFR territories.

CONCLUSIONS

At the end of treatment, a reduction in baseline MBF but no significant changes in hyperemic MBF were observed in ischemic myocardial territories, and therefore the significance of the observed improvement in CFR remains unclear.

Automated sonographic evaluation of testicular perfusion

Contrast-enhanced ultrasound (US) imaging is potentially applicable to the investigation of vascular disorders of the testis. We investigated the ability of two automated computer algorithms to analyse contrast-enhanced pulse inversion US data in a rabbit model of unilateral testicular ischaemia and to correctly determine relative testicular perfusion: nonlinear curve fitting of the US backscatter intensity as a function of time; and spectral analysis of the intensity time trace. We compared (i) five metrics based on the algorithmic data to testicular perfusion ratios obtained with radiolabelled microspheres, a reference standard; (ii) qualitative assessment of the US images by two independent readers blinded to the side of the experimental and control testes to the radiolabelled microsphere perfusion ratios; and (iii) results of the algorithmically-derived metrics to the qualitative assessments of the two readers. For the curve fit method, the algorithmically-derived metrics agreed with the reference standard in 54% to 68% of all cases. For the spectral method, the results agreed in 70% of all cases. The two readers agreed with the reference standard in 40% and 35% of all cases, respectively. These results suggest that automated methods of analysis may provide useful information in the assessment of testicular perfusion.

Pulse-Inversion US Imaging of Testicular Ischemia: Quantitative and Qualitative Analyses in a Rabbit Model

PURPOSE

To quantitatively and qualitatively assess perfusion with pulse-inversion (PI) ultrasonography (US) in rabbit model of acute testicular ischemia.

MATERIALS AND METHODS

Institutional animal care committee approval was obtained. After 35 rabbits underwent unilateral spermatic cord occlusion, testicular Doppler US and contrast material-enhanced PI imaging were performed. Enhancement data yielded perfusion measurements including mean value during the first 10 seconds, mean value over entire recorded replenishment curve, and curve slope during the first 5 seconds. Calculated perfusion ratios were compared with radiolabeled microsphere-derived perfusion ratios. Two readers assessed testicular perfusion as none, possible, or definite and relative perfusion as greater to the right testis than to the left, greater to the left testis than to the right, or as equal to both testes. With kappa statistics, interobserver agreement for all imaging methods was determined. Association between qualitative perfusion categories and radiolabeled microsphere-based perfusion measurements was assessed. Quantitative and qualitative determinations of relative perfusion were compared with radiolabeled microsphere-based measurements.

RESULTS

Correlations between calculated and radiolabeled microsphere-based perfusion ratios were determined (r=0.49-0.64). Interobserver agreement for presence of perfusion was excellent (kappa=0.76), and that for relative perfusion assessment was good (kappa=0.55). Neither kappa value varied significantly with imaging method. The percentage of times a testis classified as having definite perfusion had greater perfusion as measured with radiolabeled microspheres than a testis classified as having no perfusion or possible perfusion was higher with PI imaging than with Doppler US (85%-98% vs 72%-89%). Identification of the testis with less perfusion was better with quantitative methods than with qualitative assessment of images by the readers (75%-79% vs 34%-60%, P<.004).

CONCLUSION

PI imaging, compared with conventional Doppler US methods, provides superior assessment of perfusion in the setting of acute testicular ischemia.

Vaskuläre Bildgebung mittels kontrastverstärkter Sonographie in der experimentellen Anwendung

The possibility of employing contrast-enhanced ultrasound for sensitive detection of perfusion has resulted in new forms of application in fundamental medical biological research that go far beyond mere preclinical evaluation of these techniques. This contribution explains the methods for visualization and quantification of perfusion with contrast-enhanced sonography and provides an overview of how these functional examinations have been used to date. The procedure is generally considered indicated when information on tissue perfusion using ultrasound is required. This topic is also gaining increasing clinical interest, e.g., for assessment of myocardial, cerebral, and renal perfusion or for monitoring therapy. Among the various new treatment procedures that have been investigated in animal models with ultrasound, particularly pro-angiogenic and antiangiogenic therapy approaches predict promising new fields for application of contrast-enhanced ultrasound.

Ultrasound-mediated microbubble destruction enhances VEGF gene delivery to the infarcted myocardium in rats

OBJECTIVE

To investigate the possibility of improving the delivery of vascular endothelial growth factor (VEGF) gene to the myocardium in rats by using ultrasound-mediated microbubble destruction (UMMD).

METHODS

Fifteen male Wistar rats underwent left anterior descending coronary artery ligation in this study. The rats were divided into three groups 3 days after ligation. Ultrasound microbubble vectors (UMVs) attaching to pcD2VEGF121 gene were injected into the tail vein of rats with or without simultaneous echocardiographic microbubble destruction in two groups. The third group was used as control group. VEGF protein expression and formation of new blood vessels were evaluated by immunohistochemical technique during autopsy on 15 rats at 2 weeks after gene transformation. Microvascular density (MVD) in the area with myocardial infarction was counted under a microscope.

RESULTS

VEGF protein expression and MVD in the ischemic myocardium were higher in the rats receiving UMMD than in the group that did not receive UMMD.

CONCLUSION

UMMD is a noninvasive method to effectively improve the delivery of targeted genes to the heart.

Angiogenesis model for ultrasound contrast research

RATIONALE AND OBJECTIVES

To optimize an angiogenesis model for imaging research that is stable and can be imaged several times over the angiogenic time course.

MATERIALS AND METHODS

Mice and rats received two injections of 0.4 mL of extract of basement membrane matrix (Matrigel; Becton Dickinson Labware, Bedford, MA) in the subcutaneous spaces on either side of the spine. One of the two Matrigel plugs in each animal had either 0.1 microg/mL of basic fibroblast growth factor (bFGF) (11 mice), 1.0 microg/mL of bFGF (12 mice, 5 rats), or 1.0 microg/mL of bFGF and 60 U/mL of heparin (11 mice). Three to 12 days after implantation, animals were imaged before and after the administration of up to four injections of 0.1 mL AF0150. Phase inversion imaging was used on a Siemens Elegra (Siemens ultrasound, Issaquah, WA) equipped with a 13 MHz VFX transducer. Three observers subjectively assessed the pattern of enhancement using a four-point scale. The Matrigel plugs were then removed and two observers graded the angiogenic response on a four-point scale. Ten Matrigel plugs, five with 1.0 microg/mL bFGF and five without, were evaluated histologically following immunohistochemical staining with anti-CD31.

RESULTS

The angiogenic response was greater in Matrigel plugs with 1.0 than with 0.1 microg/mL of bFGF. Heparin did not increase the angiogenic response. Vessels were predominantly at the periphery of the plugs with variable central penetration. Plugs appeared anechoic and homogeneous on ultrasound. Contrast enhancement within the plug occurred in 44% of mice with an angiogenic response at or after day 6 and the enhancement increased with the angiogenic response. In the others, peripheral enhancement could not be distinguished from the enhancement of surrounding tissues that were also hyperemic. The thicker rat skin interfered with plug assessment.

CONCLUSION

A stable angiogenesis model without the complexity of tumors is described. This model offers the opportunity to image the development and/or inhibition of angiogenesis. Neovasculature in Matrigel was detectable using ultrasound contrast. Quantitative studies correlating the degree of enhancement to microvascular density will be determined in subsequent studies.

Validation of ultrasound contrast destruction imaging for flow quantification

Our purpose was to validate in vitro a kinetic flow model based on microbubble signal decay curve. Using a 3.5 MHz transducer and phase-inversion (1.8 MHz central transmit frequency), a renal dialysis cartridge oriented vertically was imaged in the transverse plane as 1:1000 dilution of AF0150 was infused at 50, 100, 200, 300 and 400 mL/min. Ten gray-scale images were acquired at each infusion rate using 2.5, 5 and 10 frames/s at 100%, 40%, 15% or 1% of maximum transmit power. Video-intensity measured on each 10 images was fit to a kinetic model using Sigma Plot that yielded microbubble concentration, velocity and destruction per frame. These were correlated with the experimental conditions. At 100% power, video-intensity on the first frame (microbubble concentration at equilibrium) was similar for all flow and frame rates. The model fit the experimental data for all flows at 10 frames/s and for flows lower than 400 and 100 mL/min at 5 frames/s and 2.5 frames/s, respectively. The calculated flow was similar to the experimental flow rates, regardless of technique (r(2) = 0.98). Microbubble fraction destroyed per frame was similar for all flow and frame rates and increased linearly with transmit power (r(2) > 0.98). These results suggest that using appropriate power and frame rate for a given flow rate, estimates of fractional blood volume, flow and destruction fraction can be calculated from the decay curve using 10 frames that can be acquired in 1 to 4 s.

Blood flow quantification with contrast-enhanced US: "entrance in the section" phenomenon--phantom and rabbit study

PURPOSE

To investigate changes in destruction-replenishment curves (in vitro and in vivo) that result from microbubble destruction in feeding vessels that pass through the imaging plane before microbubbles enter the region of interest (ROI).

MATERIALS AND METHODS

During continuous injections of an ultrasonographic contrast agent, nonlinear gray-scale images were obtained in vitro in the longitudinal plane of a renal dialysis cartridge flow phantom (flow rates of 100, 200, and 400 mL/min) and in vivo in the coronal plane of the left kidneys of two rabbits (two kidneys). Destruction-replenishment curves were obtained for the dialysis cartridge in ROIs located immediately after the entrance of the microbubbles into the image plane and further from the entrance, after microbubbles had traveled across the complete length of the imaging plane. Replenishment curves were also obtained from ROIs in the rabbit kidneys at the level of segmental arteries, distal interlobar arteries, and the cortex.

RESULTS

The ROIs immediately after the entrance of the microbubbles in the image plane of the dialysis cartridge and in the segmental artery of the kidney followed a typical exponential function, A(1 - e-alphat). Early portions of curves obtained in ROIs filled with microbubbles that had already passed through the image plane of the dialysis cartridge or in the renal cortex were not well described by such a function. The shape of the curve and the variations as a function of flow rate can be explained by means of a mathematical model based on indicator-dilution theory.

CONCLUSION

When the feeding vessels of an ROI travel across the ultrasound field before they reach the measurement region, the typical shape of the replenishment curve is modified (reduced velocity parameter and plateau).